Environmental factors such as health and education played a role in both the rise in IQ scores during the 20th century, as well as a long-term decline dating back to the mid-'70s, a study from Norwegian researchers suggests.

The study focuses on the Flynn effect, a term used to describe the steady rise in IQ scores during the 20th century. IQ scores started trending downward after 1975, the study shows.

Researchers with Oslo's Ragnar Frisch Center for Economic Research studied scores from more than 730,000 men between 1962 and 1991 who participated in military conscription testing. Researchers then narrowed down to focus on families with brothers to determine whether genetics or environmental factors played a greater role in IQ scores.

Results showed fluctuations in scores between brothers were similar to other participants, suggesting outside factors such as better health, changes in the education system or better standards of living, had a greater impact than family genes.

"The trends that we did find within the families are essentially the ones that we see across the population," said Ole Rogeberg, a co-author of the study.

The study was published in the journal Proceedings of the National Academy of Sciences.

Rogeberg said they can't completely rule out genetics as playing a factor in declining IQ scores, but it's potential impact isn't as significant as environmental factors. "They are not driving the large scale trends that we're seeing."